The specialty of the house is a multiflavored melange that includes all the colors of the parties of Lebanon’s political spectrum, now ostensibly united after three years of discord.

Banner in central Beirut (Photo: Simon Roughneen)

But sweet sloganeering aside, a political chill is in the air, as uncomfortable as Beirut’s summer heat. Tensions between Sunni and Shi’ite Muslims are rising, and Syria is reasserting its political clout three years after it was forced to withdraw its troops from Lebanon in the aftermath of the assassination of former Prime Minister Rafik Hariri.

Renewed fighting last week in the northern city of Tripoli, a Sunni-dominated region, underlined the precariousness of the peace agreement reached in Qatar in May between the Hezbollah-led opposition backed by Syria and Iran and the Western-supported March 14 movement, named for the start of the Cedar Revolution triggered by the Hariri assassination in February 2005.

“A lot of Saudi money has been put into the north to cultivate Wahhabi/Salafist ideology, to counter Hezbollah,” reflecting wider Sunni-Shi’ite regional rivalries, said Ahmad Moussali, a professor of political science and Islamic studies at the American University of Beirut. “These radicals see the Lebanese army as weak, and March 14 Sunnis cannot stop them confronting Shi’ites or Alawites.”

Alawites are members of a Shi’ite offshoot that is the sect of Syrian President Bashar Assad.

Saudi Arabia follows the strict Wahhabi branch of Sunni Islam. Salafism is a variant of Wahhabism practiced largely in northern Africa. Hezbollah is a Shi’ite Islamist militant group based in Lebanon.

One northern Lebanese Sunni jihadist group called Fatah-al-Islam is regarded as a Syrian creation, raising suspicions that Damascus is orchestrating both sides of the Tripoli fighting.

“The Syrians like to see this kind of instability,” said David Schenker, a Lebanon specialist at the Washington Institute for Near East Policy. “At the end of the day, it may be the only thing that saves them from the tribunal.”

A U.N. tribunal investigating the Hariri assassination has implicated the Syrian intelligence agency, but Damascus has denied any involvement and has limited its cooperation.

The sectarian tensions in Lebanon, meanwhile, are fueled by the slightest of provocations.

Dozens of bullet-marked, shelled-out buildings line the Sunni-Alawite divide between the warring neighborhoods in Tripoli. One of them is Jamal al-Rai’s garage.

He said he thinks his property was targeted because “I had the banner of Saad Hariri outside.”

Saad Hariri, Sunni head of the March 14-aligned Future Movement, is the son and political heir of Rafik Hariri, whose assassination on Feb. 14, 2005, sparked Lebanon’s Cedar Revolution a month later. Almost a third of the population took to Beirut’s streets to protest what they regarded as a hit against a political figure resisting Syrian domination.

However, the March 14 movement re-energized Hezbollah and its supporters, who quit the government in 2006 to try to block formation of a U.N. tribunal to investigate the Hariri killing. Now the tribunal, created last year, may be in jeopardy. Under the Qatar peace agreement, Hezbollah achieved long-sought veto power over government actions.

On Friday, the new “unity” government announced agreement on a policy statement defending Hezbollah’s right to keep weapons to defend the country against Israel. The statement is to be presented to parliament next week, according to the Reuters news agency.

In the meantime, Syria’s Mr. Assad is enjoying at least a temporary rehabilitation. Invited to a Mediterranean summit in Paris recently, he met for an hour and 15 minutes with French President Nicolas Sarkozy. Turkey has been mediating talks between Syria and Israel over the Israeli-occupied Golan Heights and other issues.

“The Syrians have won the political battle in Lebanon and have split the United States from France by opening negotiations with Israel,” Mr. Schenker said.

Syrian pledges to recognize Lebanese sovereignty, demarcate the border and establish an embassy in Beirut may wait, he said, in anticipation of a new Lebanese government and an international environment more friendly to Damascus.

Syria’s resurgence disheartens Sunnis, already unsettled by Hezbollah’s brief takeover of West Beirut in May and the formation of the unity government.

“We hope the international community will not allow Syria back here,” Sunni politician Bassam Khodar Agha, president of the March 14-linked Free Lebanese Movement, said in a recent interview in Tripoli. “Syria is causing unrest to facilitate Hezbollah. But we trust that the U.S. will support a democratic and free Lebanon.”

At a hearing in Washington on Tuesday, Rep. Gary L. Ackerman, chairman of the House Foreign Affairs subcommittee on the Middle East and South Asia, criticized the Bush administration for failing to provide sufficient backing to the March 14 alliance.

“There is a serious need for us to review both our overall strategy and our tactics,” the New York Democrat said. “What happened in Lebanon [in May] was absolutely foreseeable and probably preventable. The Bush administration’s response remained limited and tactical.”

Jeffrey D. Feltman, a former U.S. ambassador to Lebanon and now the principal deputy assistant secretary of state in the Bureau of Near Eastern Affairs, defended U.S. policy in Lebanon as “a long-term investment,” focusing on improving the capability of the Lebanese army “to reduce the need for the Lebanese to rely on private militias.”

Barbara Slavin and Baptiste Etchegaray in Washington contributed to this article.